Stephen d



(N0 Model.)v

S. D. FIELD. Rheostept.

Patented May 24,1881.

5000 ohms www;

' @4M/.5MM

N. PETERS. Phmc-Lilhographcr. wns'nnglon. n. c.

III/IIIA UNITED STATES PATENT OFFICE.

STEPHEN D. FIELD, OF NEW YORK, N. Y.

RH EOSTAT.

SPECIFICATION forming part of Letters Patent No. 242,092, dated May 24,1881.

(No model.)

To all whom it may concern Be it known that I, STEPHEN D. FIELD, acitizen ot' the United States, residing in the city, county, and Stateof New York, have invented certain new and useful Improvements inRheostats for Duplex Telegraphs, of which the following is aspeciication.

My invention relates to certain improvements in apparatus for thetransmission ol" independent telegraphic signals simultaneously fromopposite ends of the same line.

The object of my invention is to neutralize or prevent the production ofthe false signals which tend to be manifested upon thereceiving-instrument at the transmitting, or, as it is technicallytermed, the home, station, by the so-called static discharge,7 whichconsists in the sudden escape to earth of a quantity of electricitystored up or accumulated upon the main line by inductive action duringthe outward flow of the electric current, which takes place when atelegraphic signal is transmitted.

The invention relates more particularly to an improved construction ofthe rheostat which constitutes the equating` circuit, commonly termedthe artificial line, and it consists in forming the said rheostat of twoparallel conductors, placed in close proximity to and insulated fromeach other, the said conductors being of substantially equal length, buthaving different resistanees. These are joined together at one end toform a loop, which is inserted n the artificial line and forms theprincipal portion thereof.

In the accompanying drawings, Figure l is a diagram illustrating theprinciple upon which my apparatus is constructed. Fig. 2 is a diagramshowing the manner in which the length of the artificial circuit may bevaried. Figs. 3, 4, and 5 show different modifications in theconstruction of my apparatus.

In Fig. l I have represented one terminal station arranged for duplextransmission according to the ordinary method, together with alineextending to the earth at the distant station.

K is an ordinary open circuit or three-point key, the rear contact-stop,3, of which is conn nected directly to the earth, while the frontcontact-stop, 2, is connected to one pole of a battery, E, the otherpole of which is to earth.

m m are the two equal and opposiu helices oi' a differentialelectro-magnet, M, which actuates the receiviiig-instrument. Then thekey K is depressed and brought in contact with its front stop, 2, acurrent from the battery E passes through the key to the point 1, whereit divides, one portion going by the wire 4, through the helix m, overthe line L, to the distant station, and thence to the earth at g,returning through the earth to the opposite pole of the battery. In likemanner the remaining portion of the current goes, by the wire 5, throughthe helix m', thence through the artificial line L L2 to the earth atg', and thence returns in like manner to the other pole ot' the battery.This latter branch circuit is technically termed the artificial line inorder to distinguish it from the mainline, which eX- tends to thedistant station. It' the resistance of the articial line be so adjustedas to be approximately the same as that ot' the main line, the currenttransmitted by the key will divide at the point l into two equalportions, which will produce equal or opposite electrodynamic effectsupon the armature of the electro-magnet M, and the said armature willtherefore remain at'rest when the key Kis depressed, notwithstandingthat a current is passing over the line L to the distant station. It,however, the distant station transmits a current from its own battery(not shown) at the same time, the strength ot` the current in the mainline is augmented by the combined action of both terminal batteries, itselectro-dynamic effect overpowers that ot' the current of the artificialline, the armature of the electro-magnet M is attracted, and a signalproduced at the home station. Thus it will he understood that thereceiving-instrument at the home station responds only to currents orsignals comin g from the distant station, and not to those transmittedby the key at the home station, and consequently the two stations, whenprovided with similar apparatus, can transmit signals simultaneously toeach other without interference, the receiving-instrument at eachstation, although at all times traversed by the current ofthe main line,responding only to the signals produced by the transmitting-key at theother station.

In order to produce the result hereinbet'ore IOO set forth it isobviously essential that the resistance of the artificial line should beas nearly as possible equal to that of the main line. rlhis hasheretofore been effected by placing one or more `rheostats in theartificial line. These consist of a suitable length ol" comparativelythin wire, preferably made of some metal which is a poor conductor ot'electricity. A sufficient length of such wire is wound upon one or morespools or bobbins, and so arranged in connec tion with connnutators thatany required length ot' it may be included in the circuit of theartificial line. By this means an equal division of the current from thebattery E between the main and artificial lines may be readily broughtabout.

Having thus explained the construction and mode of operation of anordinary duplex-tele graph apparatus, I will next describe the nature ofmy present improvement, and the manner ot' its application thereto inthe best. man- {lel DOW kllOWll t0 111B.

Itis well known that an insulated telegraphic line-wire of considerablelength, whether suspended above the earth or submerged beneath thewater, is capable of accumulating or storing up a quantity ofelectricity while connected with a source ot' electricity. This propertyof an insulated conductor is termed its inductive77 or electrostaticcapacity, andthe electricity so stored up and retained is called thestatic charge77 ot' the conductor. The electrostatic capacity ot theinsulated conductor is a quantity depending upon the extent of itssuperficial area, and upon the thickness of the non-conducting spacewhich separates it from the earth, or from other conductors in electricconnection with the earth, which insulatingspace is called thedielectricl7 Thus in the case of an ordinary telegraph-liuc suspendedupon poles in the air the earth and the surrounding objects connectedtherewithsuch as buildings, trees, and the like-form the outer inductivesurface, while the air constitutes the insulating medium or dielectricsurrounding the conductor. ln the case of a submarine cable theinsulating-coating ot' guttapercha constitutes the dielectric, and theiron armor ofthe cable, or the surrounding water, asthe case may be, theouter ind uctive-surface. It will appear, therefore, from the hereinbeforementioned considerations, that when a long line of telegraph isconnected with the battery by depressing the key at the sending-station,as for the purpose of' transmitting a signal, the line will acquire aconsiderable static charge. At the completion ofthe signal, when the keyis raised, the line is first disconnected from the battery andimmediately afterward connected directly to the earth at the homestation, whereupon the accumulated induced electricity stored up in theline will suddenly escape to the earth, traversing one coil of theelectromagnet M of the home receiving-instrument, and producing what istermed the static disin which it is placed have practically noelectrostatic capacity, there will be no corresponding discharge fromthe artificial line through the opposing coil oi' the electro-magnet M,and consequently an extra or false signal ot' short duration will beproduced by the uncompensated action of the static discharge of the mainline in the electro-magnet.

I have discovered that the disturbing efi'ects of the static dischargefrom the line upon the apparatus at the home station maybe compensatedor neutralized by an improved construction of the equating-rheostatwhich constitutes the principal portion of' artificial line, by whichmethod of construction it is placed under electrical conditionscorresponding to those of th'e main line.

In Fig. l the main circuit consists ot' a lincwire, L, which we mayassume to have a total resistance of, say, tive thousand ohms, extending to the distant station, and of the earth G, which constitutes theparallel return-conduc tor, and which has little or no resistance. Nowlet the artificial line L in like manner be coustructed of a thin wirecomposed of metal of inferior conductivity, but of sufficient length tootter a resistance of five thousand ohms, and let this bejoined at thepointl to another conductor ot' very small resistance, L2, laid parallelwith it and extending to the earth at g or to the other pole ofthebattery. It is obvious that an inductive action must take place betweenthe conductors L and L2 ot' the artificial line, which will correspondin its nature to that which takes place between the main-line conductorL and the earth G beneath it, and that these effects will balance orneutralize each other in the opposing coils m and m of thereceiving-magnet M. I avail myself of" this principle in theconstruction of my improved rheostat, which consists ot' a conductor ofgreat resistance and a conductor of' little resistance, equal in length,placed parallel to each other and properly insulated. The constructionand arrangement ot' the rheostat may be modified in various ways,according to circumstances. lt may be rolled up in a flat spiral, asshown in Fig. 3, or upon a bobbin, as shown in Fig. et. I consider itpreferable, however, to lay it to and f'ro upon a plane surface, asshown in Fig. 5. The particular arrangement, however, may be varied inmany ways, so long as the principle is kept in View.

The electrostatic capacity of the open-air line L varies materially indifferentconditions ot' the weather, being much greater in cold or dryweather, when the insulation is good, than in wet or damp weather, whenthe insulation is poor. It results from this that the efiect of thestatic discharge upon the home instrument is much greater at some timesthan at others. As the artificial line is not exposed to these changesin the weather, its electrostatic capacity and the force of itsdischarge remain practically constant.1 Hence, unless some charge.77 If1the rheostat andthe artiicial line means are provided for adjustingthis, the

IOO

IOS

IlO

IIS

IZO

compensation will frequently become very imperfect. I therefore preferto arrange my rheostat in a greatery or less number of sections, in themanner indicated in Fig. 2, with peg-oommutators or other equivalentdevices placed between the sections, as shown at a, b, c, and

' d, whereby the length and resistance of the operative portions of theartificial line maybe varied at pleasure, and its electrostatic capacitylikewise varied at the same time in corresponding` ratio.

I do not desire to limit myself to the use of the rheostat constructedsubstantially as set forth exclusively in connection with the particularform of duplex telegraph hereinbefore described, as it may be employedwith equally good results in combination with ina-ny other known formsof such apparatus. The modifications necessary to adapt it to such usewill readily suggest themselves to those skilled in the art withoutfurther description.

I claim as my invention- An equating-rheostat for duplex telegraphs,constructed, substantially as hereinbefore set forth, of two parallelconductors placed in close proximity to and insulated from each other,having substantially the same length, but different resistances, andjoined together at one end to form a continuous conductor.

In testimony whereof l have hereunto subscribed niy name this 31st dayof March, A. D. 1881.

STEPHEN D. FIELD.

Witnesses MILLER G. EARL, CHAs. A. TERRY.

